The present invention relates to a video camera whose images are based on polarized light to generate images from the first several hundreds of micrometers of superficial tissue layers below a tissue surface. This superficial region is where diseased tissue (pathology) usually arises in many tissues such as the skin, gastrointenstinal tract, lungs, reproductive tract, urinary tract, biliary tract, and inner lumen of blood vessels.
The use of light in the ultraviolet-visible-near infrared wavelength range to image and characterize biological tissues is being widely pursued. These efforts have relied on several techniques. A first technique is absorption spectroscopy in which molecules electronically absorb certain wavelengths of light and hence attenuate the transmission or reflectance of that light to yield characteristic xe2x80x9cabsorption spectraxe2x80x9d. A second technique is Raman spectroscopy in which molecules vibrationally absorb certain wavelengths of light, more in the infrared, and hence attenuate transmission yielding xe2x80x9cRaman spectraxe2x80x9d. A third technique is fluorescence spectroscopy in which molecules absorb certain wavelengths of light and re-emit longer wavelengths of fluorescence yielding characteristic xe2x80x9cfluorescence spectraxe2x80x9d. A fourth technique is scattering spectroscopy, in which photons of different wavelengths are scattered differently by cells yielding xe2x80x9cscattering spectraxe2x80x9d.
Motivated by a desire to better exploit scattering spectroscopy, this method of imaging concentrates the image contrast mechanism into the upper couple hundred micrometers of tissue. This superficial layer of tissue is the region where tissue pathology arises in many tissues.
One type of light used for imaging of materials is polarized light. Polarized light is strongly reflected off the surface of a material at the air/material interface. This reflectance depends on whether the polarized light is aligned xe2x80x9cparallelxe2x80x9d or xe2x80x9cperpendicularxe2x80x9d to the plane of the material. xe2x80x9cParallelxe2x80x9d polarized light tends to bounce off the material surface. xe2x80x9cPerpendicularxe2x80x9d polarized light tends to penetrate into the material. This distinction between parallel and perpendicular alignment of polarized light is the basis of polarized lens in sunglasses which reject the parallel light reflected off a road surface.
Two approaches toward using this distinction between parallel and perpendicular light have been practiced. The first approach involves imaging material surfaces by selective acceptance of parallel polarized light. For example, polarized light has been used to detect xe2x80x9cman-madexe2x80x9d materials such as glass and metal within a field of xe2x80x9cnaturalxe2x80x9d materials such as trees, foliage, and organic soil. The second approach involves imaging material depths by selective rejection of parallel polarized light. For example, polarized light has been used to discriminate the skin surface from the skin depth. Illuminating the skin surface with parallel polarized light and viewing the skin by eye through glasses which are polarized parallel will emphasize the skin surface. Illuminating with parallel polarized light while viewing with glasses that are perpendicular polarized light will emphasize the tissue depth. In the latter case, there is always some parallel light which enters the skin but this light becomes randomly polarized by scattering within the tissue. Hence, viewing through perpendicular polarized glasses essentially rejects the surface reflectance and views the tissue depth with randomly polarized light. Imaging has been described that illuminates with perpendicular polarized light to achieve penetration of light into a tissue, then uses two wavelengths of light to enhance the contrast of a buried vessel based on absorption spectroscopy. Again, the image is based on light that penetrates deeply into the tissue and hence becomes randomly polarized. Viewing through an optical element which selects perpendicular polarized light offers a means of rejecting the glare of surface reflectance.
The task of identifying tissue pathology in the superficial tissue layers, however, is not served by either of the above. About 2-4% of the parallel polarized light is reflected by the surface. Such light does not interrogate the inner tissue where the pathology is located. About 91-93% of the reflected light is randomly polarized and is comprised of light that has penetrated deeply and been multiply scattered by the tissue. Such light is only a blinding artifact while attempting to observe the superficial tissues where pathology arises. Even observing the perpendicularly polarized light component of such multiply scattered deeply penetrating randomly polarized light does not discriminate light that scatters superficially from light that penetrated deeply. Only about 5% of the reflected light is not randomly polarized but is back-scattered by the superficial couple hundred micrometers of tissue. This invention provides a device to image based solely on that 5% of light that has penetrated the surface but not penetrated the tissue depth.
The present invention relies on taking a set of measurements using a broad illumination beam of light circularly polarized or linearly polarized at different angles of alignment and observing the tissue with a system that discriminates circularly polarized light and the various alignments of linearly polarized light. Also, a number of wavelengths of light are used to acquire images. The choice of wavelength may be made by the choice of light source or by including filters at either the source or camera detector. The wavelength dependence of polarized light scattering depends on the size distribution of tissue ultra-structure, i.e., cell membranes, protein aggregates, nuclei, collagen fibers, and/or keratin fibers. A set of images is taken with different combinations of source and collector polarization and wavelength. The images are then recombined to yield an image which rejects surface reflectance, rejects deeply penetrating light, and is optimally sensitive to just the light reflected from the superficial layer of the tissue. A set of images may also be taken with different combinations of source position and/or collector position. The individual images may be acquired by moving a single camera or source to different locations, by using several different cameras or sources, or by using mirrors or lenses to capture images from different angles or tissue locations.
The invention may include an optical element in contact with the tissue surface (e.g., a glass flat), an oblique angle of source illumination, and an angle of camera observation which differs from the angle of surface reflectance. The glass flat provides a tissue/glass interface that is well coupled and smooth such that oblique incidence of illumination light will cause surface reflectance to reflect at an oblique angle opposite the incident angle of illumination. The camera views the surface at an angle different from this angle of surface reflectance and hence no surface reflectance enters the camera. In another embodiment, the camera and the illumination source may each be positioned at the same angle with respect to the tissue surface wherein a beam splitter is used to facilitate the use of a single location for the illumination source and the camera, and wherein the angle of the camera and the illumination source is different than the glare angle.
For example, consider a system where linearly parallel polarized light is used for illumination and two images are acquired, one image selecting linearly parallel (Par) polarized light (i.e., parallel to the light source-tissue-camera plane) and one image selecting linearly perpendicular (Per) polarized light (i.e., perpendicular to the light source-tissue-camera plane). The two images are recombined using the following expression:
New image=Parxe2x88x92Perxe2x80x83xe2x80x83(Equation 1)
Each Par and Per image includes about 90% of the corresponding parallel or perpendicular component of randomly polarized light from deeper tissue layers and these components are equal in magnitude. Hence, the difference Parxe2x88x92Per subtracts these common contributions from deep tissue layers. The surface reflectance (or glare) is rejected by the strategy of oblique incidence of illumination and the optical element in contact with the tissue to ensure glare is diverted from the camera. Hence the Parxe2x88x92Per image is based on the 5% of the total reflected light which is back-scattered from only the superficial tissue layer.
Another example of how to recombine polarized light images to achieve optimal sensitivity to the scattering by the superficial tissue layer is to reject any interference due to superficial pigmentation that absorbs light. For example, a doctor cannot see the superficial tissue layer beneath a freckle or beneath (or within) a pigmented nevus. The following expression is useful:
New image=(Parxe2x88x92Per)/(Par+Per)xe2x80x83xe2x80x83(Equation 2)
The numerator as before selects the light scattered from the superficial tissue layer. The denominator provides a means of rejecting the influence of a superficial layer of absorption such as the melanin in the epidermis of skin. Melanin is the absorbing pigment of skin and acts as a filter on the tissue surface. All light must pass this filter twice, once on entry and once on exit. This filter attenuation is a common factor in all images acquired. Hence, by taking the ratio in Equation 2, the common factor cancels. In the image, the melanin disappears. For example, a pigmented freckle will disappear or the pigment of nevi will disappear. Hence, one can visualize the polarized light scattered from the superficial tissue layer without interference from superficial pigmentation.
The present invention has also found that using incoherent light, as opposed to coherent laser light, allows images which are free from xe2x80x9claser specklexe2x80x9d which is the interference of scattered coherent light. Such speckle is an interference that confuses the imaging of the superficial tissue layer. Lasers with very short coherence lengths ( less than  less than 100 (um) qualify as an xe2x80x9cincoherentxe2x80x9d light source for such imaging. However, a coherent light source, such as a laser, may also be used as the light source in the present invention. The use of a coherent light source provides another means of characterizing the type of reflected light and hence characterizing the tissue. The use of interferometric techniques also provides a variety of new measurements. Additionally, modulated illumination light techniques in which the intensity of the illumination light is modulated by a periodic waveform or by an encoded bit stream may also be utilized.
Accordingly, an object of the present invention is to provide an imaging device capable of generating an image using light scattered only by the superficial layer of a tissue.
Another object of the present invention is to provide an imaging device capable of rejecting light reflected from the surface (surface glare).
Yet another object of the present invention is to provide an imaging device capable of rejecting light reflected from deep tissue layers (randomly polarized light).
Another object of the present invention is to provide an imaging device capable of acquiring a set of images based on different combinations of circularly and linearly polarized light for illumination and collection.
Another object of the present invention is to provide an imaging device capable of acquiring a set of images based on different choices of wavelength of light for either illumination or collection.
Another object of the present invention is to provide an imaging device capable of recombining the acquired set of images.
Another object of the present invention is to provide an imaging device capable of recombining acquired images in order to cancel the influence of absorbing superficial pigmentation.
Still a further object of the present invention is to provide an imaging device wherein multiple illumination source and/or camera positions are utilized to acquire a set of images.
Yet a further object of the present invention is to provide an imaging device wherein multiple light techniques are utilized to acquire a set of images.
Another object of the present invention is to provide an imaging device wherein a beam splitter or equivalent optical element (e.g., diffractive optics) is utilized to allow the illumination source and the camera to be positioned along a co-linear light path.